This invention relates to a low cost, high performance instrumentation package particularly applicable, but not limited to radiation detectors used in solid mineral mining or oil well logging operations (referred to in the industry as wireline operations), and to a highly integrated electro-optical radiation detector for Measurement While Drilling (MWD) or Logging While Drilling (LWD) operations in which the shock and vibration levels are more extreme.
Scintillation packages typically include a scintillation or crystal element (or simply, crystal) supported within a cylindrical shield. A consequence of making scintillation packages sufficiently rugged for reliable use in harsh environments, such as coal mining or oil well logging, has been some reduction in performance. Making scintillation packages rugged has also increased the cost of fabrication. Although significant progress has been made in recent years in the development and deployment of scintillation packages which are able to withstand temperatures of up to about 200° C. and to simultaneously withstand shock and vibration, there remains a need to further improve functional performance and reduce cost without compromising reliability while operating in harsh environments.
Several factors contribute to reduced performance of scintillation packages. Thicker glass windows are sometimes used in order to withstand the thermal and shock loads and to ensure a good hermetic seal. Thicker couplers are sometimes used in order to provide cushioning between the scintillation element and the window and to provide more compliance so that the scintillation element will be less likely to separate from the coupler under vibration. These thicker elements in the optical path result in loss of light and reduce the field of view of the photomultiplier that is receiving the light from the package.
Compressive forces around a scintillation element are necessary in order to minimize movement of the element under vibration and shock since such motion will result in unwanted noise being generated in the optical output. Such compressive forces, when applied to some reflector materials around the scintillation element cause the material to become less reflective and may cause the material to partially “wet” the surface of the scintillation element. An example of a highly effective reflector material is a special TEFLON® tape, which is porous. This is one of the most effective reflectors of scintillation-produced light in the near UV range and it has been widely used for reflectors on scintillation packages for decades. This material is very pliable, however, and if pressed against the surface of a scintillation element, the efficiency of this tape, as a reflector, diminishes. Yet, a scintillation element must be held firmly in order to be used in harsh environments in order to not break and in order to not produce spontaneous scintillations. Various elastomeric materials—pottings, boots, powders and metallic elements—have been used to support scintillation elements. Each material has its own advantages and disadvantages and may be chosen accordingly. However, all known methods of support within scintillation packages using Teflon® tape and certain other reflectors, which have proven to be effective to protect the scintillation element from thermal effects and high temperature, result in compression of the tape, causing loss in performance. For example, a boot placed around the reflector must be installed in a compressed state in order to prevent movement of the element at ambient temperatures. At elevated temperatures, the scintillation element expands and increases the compressive force. The boot material, typically made from an elastomeric material, also expands to further increase the pressure on the reflector. Using properly designed metallic supporting elements improves this loading problem by limiting maximum pressure on the tape.
In a similar manner, use of porous reflective tape at the rear of the scintillation element may also be affected by the constant pressure of the rear spring.
Use of relatively thick stainless steel housings, or shields, around the scintillation element has increased attenuation of gamma radiation as it passes into the package.
Similarly, several factors contribute to increased cost of ruggedized scintillation packages. This is particularly true for designs that are directed toward minimizing the performance weaknesses described above. Replacement of thick glass windows with thinner sapphire windows is more costly because sapphire is more costly than glass and can be significantly more costly if processed in only small quantities. Use of titanium reduces attenuation below that of stainless steel, while maintaining high strength, and, for the same strength, can be thinner than aluminum. But use of titanium for shields is more costly than stainless steel. Not only is titanium metal more expensive, titanium pipe sizes and tubing sizes that are readily available in the industry for use in manufacturing shields increase the work required to perform the machining of the shields to the required tolerances. Given all the variables, making special mill runs of extruded titanium pipe in order to minimize machining costs is not commercially acceptable in most cases. Therefore, there is a need for a design approach that addresses both technical and commercial considerations.
Another factor that constrains cost cutting is that there has been a degree of standardization in the industry that places specific requirements on the external and internal dimensions of many of the commonly used configurations. These factors and others, when taken together, result in overall higher costs while providing less than optimum functional performance.
Similarly, with regard to complete detector assemblies such as radiation detectors (typically including a scintillation package coupled by a window and/or optical coupler to a photomultiplier tube or PMT), used in mining operations (both oil well drilling and solid mineral mining), and in oil well MWD and LWD operations, is that the detector be able to survive harsh environments such as high vibration and shock. For much of the history of using nuclear radiation (e.g., gamma ray) detectors for mining applications, the critical elements inside the detectors as well as the complete detectors inside a tool housing, have been supported with elastomeric materials, sometimes in combination with longitudinally placed springs. Such relatively soft elastomeric materials are often still used to provide cushioning or dynamic isolation from shock and vibration. In recent years, the technology has advanced to the use of metallic support devices which are effective and use less space. Metallic supports are described in, for example, U.S. Pat. Nos. 5,742,057; 5,796,109; 5,962,855; and 6,355,932.
As already mentioned, one element commonly found in a nuclear detector, particularly when a hygroscopic scintillation crystal is used, is a window arranged between the scintillation crystal and the PMT or other device that converts the scintillations to electrical pulses. This window is utilized because the hygroscopic crystal must be encased in a hermetically sealed enclosure, and the window allows the scintillation pulses to pass from the enclosure to the PMT. Thick glass windows are typically used in scintillation packages for use in downhole applications in order to provide needed strength and in order to provide a good hermetic seal.
In an attempt to improve detector performance, configurations have been devised for eliminating the window by placing the PMT inside a hermetically sealed housing with the crystal. An immediate advantage is that the light can pass directly from the scintillation crystal, through a thin optical coupler, into the PMT, without having to pass through the window and an additional optical coupling. Without the additional two elements in the light path, less light is lost and the amount of light from each scintillation pulse is brighter, thus increasing the gain of the detector. This increased gain, in turn has the particularly important benefit of maintaining the overall gain of the detector above a minimum level as the gain of the PMT drops due to degradation of its photocathode over time at high temperature. The pulse height resolution can also be improved by deletion of the crystal window and also the space that would have been used by the window can then be used to add more crystal. However, most previous designs that place the PMT and crystal into a single hermetic housing cannot survive in a harsh environment, such as in mining operations, and particularly in MWD and LWD operations.
Another attempt to satisfy current needs has been to place both a PMT and crystal inside a hermetically sealed housing, and then to dynamically support the PMT and crystal with an elastomeric material. This arrangement is described in U.S. Pat. No. 6,222,192. Due to the well understood need to prevent bending of the PMT/crystal, an elastomeric cylindrical sleeve is installed around the PMT/Crystal so as to rigidize the PMT/Crystal assembly. This arrangement creates additional problems, however, and has not proven to be fully satisfactory for reasons that will be better understood later.
In addition to considerations related to the internal construction of a radiation detector, it is common practice to also use elastomeric materials externally of the detector, i.e., to support the detector in the tool cavity or housing into which it must be installed. Again, elastomers take up valuable space and do not provide the high damping properties that are desired for dynamic isolation from induced vibrations. Moreover, the elastomeric support will tend to have a low resonant frequency that, when combined with the relatively low damping, will produce a relatively high response thus amplifying the vibrations induced into the detector. These amplified vibrations increase the chance of producing noise in the detector output, or of even breaking the crystal or its interface to the PMT. Elastomeric material is incapable of providing support that exhibits the properties of a hard mount during normal or expected vibration levels. Thus, there is a need for a more effective and space efficient means of supporting the improved nuclear detector within the tool in which it is to be used.
A need also remains for a reliable, compact, high performance detector that can operate for longer periods of time at high temperatures, up to 175 Deg. C. or higher while under high vibration and shock conditions.